Most of today's diesel engines are turbocharged, and more and more gasoline engines are being turbocharged. The benefits of turbocharging (reduced engine size and weight, improved fuel economy, increased power density, and reduced emissions) are well known. Tightening emission standards, high fuel-economy requirements, and end user demand for drivability require modern turbocharged gasoline and diesel engines to operate over a wider flow range, or in other words, to have a wider compressor map. There is ever-increasing demand for low-end torque and high power requirements, and thus need for compressor stages having greater map-width at high pressure ratio (3.0 and above).
Compressor stages in which wheels operate in a regular housing exhibit limitations in either stability at high pressure ratio or maximum flow capacity. Ported-shroud compressor housings are known to improve map width at high pressure ratios, but typically they bring a penalty of increased blade pass source acoustics level. Often the increased source acoustics level makes engine/vehicle system blade pass noise level unacceptable in production passenger vehicles. The increase in the source acoustic level can be as high as 15 dB as compared to a regular compressor housing. With such increased source noise and ever-diminishing engine noise, the gap between the turbocharger-related tonal noise and engine/vehicle system background noise widens even more and makes the turbocharger-related noise level unacceptable in production passenger cars.